RHEOLOGICAL MODELING OF MODIFIED ASPHALT BINDERS AND MIXTURES
Menglan Zeng, Hussain U. Bahia, Huachun Zhai, Michael R. Anderson, Pamela Turner
ABSTRACT
Rheological properties of asphalt binders and mixtures provide a fundamental understanding of the behavior of these materials. These properties play important roles in the evaluation and selection of paving materials, and in the analysis and design of asphalt pavements. Modeling of the rheological properties of paving materials has been the subject of many studies. A literature review indicates that most of the previous studies were focused on asphalt binders. The models were developed largely based on test data on unmodified asphalt materials. In the case of dynamic loading, modeling the effect of strain level has not been commonly considered. More importantly, a universal model that is applicable to both binders and mixtures has not been made available.
A model is developed in this study for the phenomenological characterization of modified asphalt binders and mixtures with modified binders under dynamic shear loading in ranges of frequency, temperature and strain of interest. It is composed of four formulations for the (magnitude of) complex modulus master-master curve, phase angle master-master curve, temperature dependency and strain dependency. This model is capable of modeling the behavior of both asphalt binder, as a viscoelastic fluid, and asphalt mixture, as a viscoelastic solid, in a universal form.
In the model, the complex modulus master-master curve is described by a generalization of the power law. The phase angle master-master curve is expressed with an empirical equation based on an examination of test data. The phase angle expression in the case of binder offers an inflexion in the relationship between logarithmic frequency and phase angle, which has been observed in modified asphalt systems. In addition, the parameters for the phase angle are independent of those for the complex modulus, thus avoiding possible oversimplification at intermediate frequencies between the extremes. While the WLF equation is used for temperature shift factor, this equation is utilized for strain shift factor. The model developed in the study represents a complex case in characterizing viscoelastic materials with reduced forms as special cases for other loading modes.
Analyses on test data for nine modified asphalt binders and 36 mixtures with the nine binders and four types of aggregate indicate that the model fits the measurements very well. Further analyses of the modeling results are performed for the sensitivity of the material properties to temperature and frequency as examples. In addition, the relationship between binder and mixture properties is examined using the modeling results, followed by an approximation of the relationship.